Creating displacement maps from 3D geometry?
There are ways of baking scalar/vector displacement maps from non-planar originals, in Blender, by using an intermediate bake of high-res shading-point positions and subtracting low-poly positions from them in a shader, using a  corresponding UV map. But that's fiddly, and complicated.
As far as I can see, your sculpt is of surface details. (Mainly dinks from rim-shots?)
I'm assuming either you want a low-poly final, in which case your height-map will be used as a bump texture (the geometry would not support displacement), or you want low-poly geometry, and the renderer to generate the subdivision for true displacement...
In either case, I think it would be easier, for this kind of detail, to use  a low-poly mesh in the first place, without going through an all-round sculpt? You could texture-paint straight into a height-map, (while seeing the results), use and manipulate ready-made scratch-maps, or, if you're looking for paricular 3D characteristics, sculpt into a flat surface, bake that, chop bits of the image out to use in collage in your UV, or as texture-paint-brushes... any combination of these things, or others not mentioned here?